/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Copy all attribute-layer values from one triangle to another, using the IndexMaps to track and re-use shared attribute values.
void CopyAttributes ( int FromTriangleID, int ToTriangleID, [FMeshIndexMappings](API\Runtime\GeometryCore\DynamicMesh\FMeshIndexMappings) & IndexMaps, [FDynamicMeshEditResult](API\Runtime\GeometryCore\FDynamicMeshEditResult) & ResultOut )